Double-worm extruders generally comprise an elongated housing formed at one end with an inlet for the thermoplastic material or compositions containing plastic materials which are to be masticated and subjected to mixing and shear in the extruder by a pair of worms received within respective worm chambers extending the length of the housing. At the opposite end of the housing, a liquefied or plastified material is extruded by the worm.
The extruder housing has its worm chambers formed therein so as to constitute a figure eight configuration in cross section.
There have been increasing efforts in recent years to augment the proportion of mineral or oxidic fillers in thermoplastic compositions.
For example, 50% or more of a composition to be subjected to thermoplastification in the extruder may be composed of such fillers. Indeed, it is possible to provide compositions in which the thermoplastic material is present only as a binder, for example in an amount up to about 20%, while the balance of the composition is abrasive mineral and oxidic substances. These compositions may be used in the fabrication of synthetic-resin-bonded ceramics.
All such material compositions are highly abrasive and the operation of the double-worm extruder can give rise to significant deterioration of the surfaces of the chambers exposed to the action of the material. It is possible to reduce the wear and abrasive attack of the material processed by coating the surfaces of the chambers with a hard-phase layer or so-called hard-facing material.
To permit the hard facing of the chambers of the extruder, generally the extruder housing is composed of a plurality of parts which are joined together along longitudinally-extending seams. For example, the housing may comprise two halves which are joined together by electron-beam welding along a longitudinal median plane. The result, of course, is that such housings have undesirable weld seams which are particularly sensitive, for example, to the high pressures which must be generated in the extruder.
Other techniques, apart from hard-facing using welding technology, for example, centrifugal coating, may also be used to apply the hard-phase layer to the body of the housing. Such techniques have been found to be expensive to use, largely because of the extensive machining required of the cast or forged body.
In the case of facing techniques, for example, it is necessary not only to accurately machine the chamber walls, but also to polish them. Indeed, the harder the hard-phase layer, the more expensive and complex are the machining techniques required to provide the chamber walls.